The present invention relates to an oscillator circuit, and more particularly, to a multiphase triangular wave oscillator circuit used, for example, in a PWM switching regulator.
The increase in the operating speed of recent central processing units (CPUs) has increased the current consumed by a CPU. A switching regulator, which supplies current to the CPU, must have a large current output and high efficiency. The switching regulator goes ON and OFF at a high speed in response to a triangular wave signal, which is generated by a triangular wave oscillator circuit, to generate a pulse signal. Further, the switching regulator smoothens the pulse signal and generates a DC voltage.
To increase the current output from the switching regulator, a plurality of regulators may be connected parallel to one another (i.e., configure multiple channels) so that the output of the regulators (pulse signals) are synthesized. A ripple current, which is produced at the input of the switching regulator, decreases the efficiency of the regulator. It is thus required that multiple channels be configured in the triangular wave oscillator circuit, which generates the triangular wave, to prevent the efficiency from decreasing.
FIG. 1 is a schematic circuit diagram of a prior art triangular wave oscillator circuit 50. The oscillator circuit 50 includes a first current source 51, a second current source 52, a charge/discharge switching circuit 53, a capacitor CT, a switch SW, a first op amp 54, and a second op amp 55.
A first current I1 flows from the first current source 51 in accordance with the power supplied from a power supply VD. The first current source 51 is connected to the ground GND via the capacitor CT and connected to the second current source 52 via a switch SW. The second current source 52 is connected to the ground GND. A second current I2 flows from the second current source 52 in accordance with the power supplied from the power supply VD. The drive current value of the second current source 52 is two times greater than that of the first current source 51.
The charge/discharge switching circuit 53 includes a first comparator 53a, a second comparator 53b, and a flip-flop 53c. The charge/discharge switching circuit 53 opens and closes the switch SW in accordance with a voltage Vn1 at a node N1 between the first current source 51 and the capacitor CT to generate a switching signal, which opens and closes the switch SW.
The first comparator 53a has a non-inverting input terminal supplied with the node voltage Vn1 and an inverting input terminal supplied with a first reference voltage VrH. The first comparator 53a generates a first comparator signal at a high level when the node voltage Vn1 becomes greater than or equal to the first reference voltage VrH.
The second comparator 53b has an inverting input terminal supplied with the node voltage Vn1 and a non-inverting input terminal supplied with a second reference voltage VrL. The second comparator 53b generates a second comparator signal at a high level when the node voltage Vn1 becomes less than or equal to the second reference voltage VrH.
The flip-flop 53c has a set signal input terminal S, which receives the first comparator signal, and a reset signal input terminal R, which receives the second comparator signal. When the first comparator signal is high, the flip-flop 53c generates the switching signal SQ so that the switch SW is closed. When the second comparator signal is high, the flip-flop 53c generates the switching signal SQ so that the switch SW is opened.
In the oscillator circuit 50, when the switch SW is opened, the first current I1 charges the capacitance CT and increases the node voltage Vn1. When the node voltage Vn1 becomes greater than or equal to the first reference voltage VrH, the first comparator signal of the first comparator 53a goes high. In response to the high first comparator signal, the flip-flop 53c closes the switch SW.
As a result, the closed switch SW causes the second current I2 to flow from the second current source 52. The current value of the second current I2 is two times greater than that of the first current I1 (I2=2xc3x97I1). Accordingly, discharge current (I2-I1) flows from the capacitance CT to the ground GND. This decreases the node voltage Vn1. The drive current value of the second current source 52 is two times greater than that of the first current source 51. Thus, the rate at which the node voltage Vn1 increases is equal to the rate at which the node voltage Vn1 decreases.
When the node voltage Vn1 becomes less than or equal to the second reference voltage VrL, the second comparator signal of the second comparator 53b goes high. The high second comparator signal resets the flip-flop 53c and inverts the switching signal SQ. The inverted switching signal SQ opens the switch SW. As a result, the first current I1 charges the capacitance CT and increases the node voltage Vn1 again.
The oscillator circuit 50 repeats such operation to generate a triangular wave signal Vct, which varies between the first reference voltage VrH and the second reference voltage VrL.
The first op amp (inverting amplification circuit) 54 has an inverting input terminal, which is supplied with the node voltage Vn1 via a resistor R4, and a non-inverting input terminal, which is supplied with a third reference voltage Vtha. The first output signal VA of the first op amp 54 is returned to the inverting input terminal via a resistor R5. The first output signal VA has a voltage obtained by inversely amplifying the node voltage Vn1 in accordance with the third reference voltage Vtha.
The second op amp (non-inverting amplification circuit) 55 has a non-inverting input terminal, which is supplied with the node voltage Vn1, and an inverting input terminal, which is supplied with a fourth reference voltage Vthb via a resistor R6. The second output signal VB of the second op amp 55 is returned to the inverting input terminal via a resistor R7. The second output signal VB has a voltage obtained by amplifying the node voltage Vn1 in accordance with the fourth reference voltage Vthb.
The resistance values of the resistors R4-R7 are set so that the amplifying rates of the first and second op amps 54, 55 are virtually the same. The third and fourth reference voltages Vtha, Vthb are set at a median voltage between the first and second reference voltages VrH, VrL ((VrH+VrL)/2)). Accordingly, the phase of the first output signal VA is the same as that of the triangular wave signal, and the phase of the second output signal VB is opposite to that of the triangular wave signal Vct.
The first and second output signals VA, VB, which have difference phases, alternately activates and inactivates two output transistors. This decreases the ripple current generated at the input of a switching regulator. As a result, the current output of the switching regulator increases, and the efficiency of the switching regulator increases.
To further increase the current output and efficiency of the switching regulator, a triangular wave having three or more phases must be generated. The prior art oscillator circuit 50 can generate two triangular wave signals (first and second output signals VA, VB) having opposite phases. However, the configuration of the oscillator circuit 50 becomes complicated when a triangular wave signal having multiple phases (three or more phases) must be generated. Therefore, the generation of a triangular wave signal having three or more phases is difficult.
It is an object of the present invention to provide an oscillator circuit that efficiently generates a triangular wave signal having multiple phases.
To achieve the above object, the present invention provides an oscillator circuit including a plurality of capacitors, each having two terminals and having a voltage between the two terminals. The plurality of capacitors includes a first capacitor. The oscillator circuit includes a plurality of first current sources, a plurality of second current sources, and a plurality of switches. Each of the first current sources charges an associated one of the capacitors. Each of the second current sources discharges an associated one of the capacitors. Each of the switches is connected between an associated one of the first current sources and an associated one of the second current sources. A plurality of charge/discharge switching circuits are connected to the first capacitor. Each of the charge/discharge switching circuits generates a switching signal for an associated one of the switches to control the charging and discharging of the associated capacitor. The switching signals of the charge/discharge switching circuits have different phases. Each of the charge/discharge switching circuits receives a first capacitor voltage between the terminals of the first capacitor and compares the first capacitor voltage with a first reference voltage and a second reference voltage to generate the switching signal that has a predetermined phase. A triangular wave signal is generated at one of the two terminals of each of the capacitors. The triangular wave signals have different phases.
A further perspective of the present invention is an oscillator circuit including first, second, and third capacitors, each having two terminals and having a voltage between the two terminals. The oscillator circuit includes a plurality of first current sources, a plurality of second current sources, and a plurality of switches. Each of the first current sources charges an associated one of the capacitors. Each of the second current sources discharges an associated one of the capacitors. Each of the switches is connected between an associated one of the first current sources and an associated one of the second current sources. A first charge/discharge switching circuit is connected to the first capacitor to generate a first switching signal that shifts the first capacitor between a charging state and a discharging state. A second charge/discharge switching circuit is connected to the first and second capacitors to generate a second switching signal that shifts the second capacitor between a charging state and a discharging state. A third charge/discharge switching circuit is connected to the first and third capacitors to generate a third switching signal that shifts the third capacitor between a charging state and a discharging state. The first to third switching signals of the charge/discharge switching circuits have different phases. Each of the first to third charge/discharge switching circuit receives a first capacitor voltage between the terminals of the first capacitor and compares the first capacitor voltage with a first reference voltage and a second reference voltage to generate the corresponding switching signal that has a predetermined phase. A triangular wave signal is generated at one of the two terminals of each of the capacitors. The triangular wave signals have different phases.
A further perspective of the present invention is an oscillator circuit for generating a plurality of triangular shape signals having different phases. The oscillator circuit includes a plurality of capacitors having output nodes, a plurality of first current sources, and a plurality of second current sources. Each of the first current sources charges the corresponding capacitor via a corresponding one of the output nodes. Each of the second current sources has current supply capacity larger than that of each of the first current sources and discharges the corresponding capacitor via a corresponding one of the output nodes. A plurality of switching control circuits are coupled to one of the output nodes in common and generate switching signals having different phases. The oscillator circuit further includes a plurality of switches. Each of the switches is coupled between a corresponding one of the output nodes and a corresponding one of the second current sources. Each switch is controlled in response to a corresponding one of the switching signals.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.